This invention relates to semiconductor switching circuits and more particularly to semiconductor switching circuits in switching power converters.
In all switching power converters, it is important to accurately coordinate operation of the power stage switching devices. Converters employing switching devices with intrinsic turn-off capability, generally require that no conduction overlap is allowed. In practice, some underlap is usually employed. To avoid the buildup of high voltages across the switching devices, the underlap is usually kept short.
In practical power converters employing bipolar devices, such as thyristors, transistors or gate turn-off (GTO) thyristors, switching delays due to charge storage must be addressed. At turn off, the storage delays are current dependent and may exceed the maximum allowable underlap. A fixed gap between consecutive switching control signals will not always be satisfactory.
Since the turn on delays are relatively short, switching overlap can be avoided if turn off or recovery of a switching device is detected and then used immediately to initate firing of the next device. This technique is illustrated in U.S. Pat. No. 4,342,076, issued July 27, 1982 to Rosswurm et al. wherein transistor switch current is sensed by a change in voltage level of a secondary winding of a transformer in the base drive circuit and completion of turn off of the transistor is used to produce a signal which initiates turn on of another associated transistor.
While this approach is satisfactory in some applications, it does not provide a continuous indication of the conduction status of each switching device which can be used for system diagnostic purposes.